1. Field of the Invention
This invention relates to a brake pressure control apparatus for controlling a brake pressure for wheel brakes of a vehicle.
2. Description of the Prior Art
Well-known apparatus for brake pressure control adopt a system of antilock control or traction control for wheel brakes of a vehicle using an auxiliary hydraulic power source for constantly keeping a high pressure to be applied as an auxiliary hydraulic pressure. It is also known to use a hydraulic booster making use of an auxiliary hydraulic pressure from an auxiliary hydraulic power source in place of a conventional vacuum type booster. Such a hydraulic booster can be easily made to be small-sized and to have a high multiplying factor and it can be combined with a master cylinder and an antilock control device to form a unitary body, which can be easily attached to a vehicle.
It is known that in the case of using a hydraulic booster, a system is adopted in which pressure is applied to a specified wheel brake by using a dynamic fluid having a boosted pressure created in a boost chamber of the hydraulic booster, not using a static fluid through a master cylinder as in other conventional systems. For example, Japanese Patent Laying-Open No. 104449/1982 discloses a pressure control mechanism of a brake operating cylinder, in which an antilock device associated with the dynamic fluid comprises an intake valve to be operated for communication between a boost chamber and a specified wheel brake, and a discharge valve for discharging the dynamic fluid from the wheel brake to a reservoir. In such a manner, the master cylinder does not move beyond a specified distance in the stroke in a repressurization process of antilock, as is different from the case of using the static fluid and thus the antilock device for the dynamic fluid can be constructed in a very simplified manner.
The brake system using the conventional vacuum type booster involves a disadvantage that a leg-power needs to be increased when a failure occurs in a front wheel brake circuit to which the greater part of the brake force is applied. In order to eliminate the disadvantage, a brake system using a hydraulic booster as described above uses a tandem type master cylinder, for example, in the case of a front-engine front-wheel drive car, in which pressures of the two chambers of the master cylinder are applied to the respective front wheel brakes and the pressure through the booster is applied to the rear wheel brakes. In the case of a front-engine rear-wheel drive car, a single type master cylinder is used, so that the pressure through the master cylinder is applied to the front wheel brakes and the pressure through the booster is applied to the rear wheel brakes.
However, in the above described brake system using the hydraulic booster, it becomes impossible to apply the pressure to the rear wheel brakes when the auxiliary hydraulic power is no longer applied as a result of a failure in the auxiliary hydraulic power source, or when a failure occurs in the rear wheel brake circuit. In such a case, the boosting function of the booster is lost and a leg-power necessary for a sufficient working of the brakes should be increased. As a result, vehicles to which the above described brake system using the hydraulic booster is applicable are limited to those having light weight.
To solve the above described problems, it may be considered to use a method for ensuring the boosting function by cutoff of a fluid path between the boost chamber and the rear wheel brakes when a pressure switch for monitoring the pressure of the auxiliary hydraulic power source detects lowering of the auxiliary hydraulic pressure due to a failure in the rear wheel brake circuit. However, even in such a method, it becomes impossible to apply pressure to the rear wheel brakes, when a failure occurs in the auxiliary hydraulic power source itself and the booster is not operated normally.